The Victorians were wild about Japanese knotweed, with its beautiful heart shaped leaves, delicate creamy-white flowers and hardy attributes which made it ideal for lining pathways and supporting banks.
Brought to the UK in the mid nineteenth century by German-born botanist Phillipp von Siebold and delivered to Kew Gardens in a box of 40 Chinese and Japanese plant varieties, knotweed spread rapidly across the UK as keen gardeners obliviously shared cuttings and disposed of garden waste.
What is Japanese Knotweed?
An extremely resilient plant thanks to large energy stores in its root system, Japanese knotweed is very difficult to kill when fully grown.
The source of its resilience lies in its native habitat. It was dug up from volcanic ash near Nagaski, Japan, where it thrived amid lava and poisonous gases thanks to its extensive network of underground stems, or rhizomes, that were able to suck up the limited nutrients.
What is the problem with Japanese Knotweed?
Now we’ve come to understand the significant damage it can do to buildings and the thousands of pounds it can wipe off the value of our homes if left untreated.
Number one on the Environment Agency’s list of the UK’s most invasive plant species, Japanese knotweed is described as “indisputably the UK’s most aggressive, destructive and invasive plant”, costing an estimated £166 million each year to treat.
Growing up to 3 metres in height, it spreads rapidly and can push up through asphalt, cracks in concrete, driveways, cavity walls and drains in its quest for light and water.
According to a YouGov survey commissioned by
Environet, most of us (78%) have heard of Japanese knotweed, but there is still a great deal of confusion about exactly what it is, what are the risks and how it can be treated successfully.
Interestingly, Japanese knotweed is not a problem in Japan where it has natural enemies in the form of bugs and fungi, but here in the UK it is unfortunately predator free.
There are trials going on to introduce Japanese psyllids, or insects, that feed on knotweed to the UK, which could help to limit its rapid spread across the country, but these are in early stages, and it isn’t known yet if the bugs will survive and thrive through our harsh winters.
How to spot Japanese Knotweed
What we do know is that the earlier Japanese knotweed is discovered the better, as it is easier to treat before the root system has matured.
Appearance
So how can we spot it? In the autumn, the plant’s growing season draws to a close. The bamboo-like canes turn brown and brittle, which can fool people into believing the plant has died or that their own efforts at treating it have been successful.
However, like many plants, the rhizome system beneath the ground is healthy and lying dormant throughout the colder winter months.
In spring, red or purple asparagus-like shoots will appear, quickly turning into green bamboo-like stems which grow at a rapid rate, several inches each day, reaching up to 3 metres in height.
Knotweed is fully grown by early summer and flowers in late summer, when clusters of spiky stems become covered in tiny creamy-white flowers. The leaves are luscious green in colour, flat and heart shaped in appearance.
How to remove Japanese Knotweed
Attempting to deal with knotweed by cutting it down repeatedly, burning it, burying it, or using common weed killers simply won’t work, as the plant can lie dormant beneath the ground, only to strike again when people least expect it.
Herbicide treatments
Professional herbicide treatments remain a very popular control method as they’re the least expensive, typically consisting of four treatments spread over two growing seasons, with a monitoring visit in the third year to check for any regrowth.
However, it isn’t suitable if the ground is going to be disturbed and isn’t particularly environmentally friendly either, as it does involve the use of strong chemicals to kill the plant.
Excavation
Excavating the area that the Japanese Knotweed is growing is the most effective way of removing the plant. It is important to be aware that the disposal of the Japanese Knotweed will also need to be arranged.
A much more environmentally friendly method is to dig out the knotweed, physically removing it from the ground.
Environet’s Resi DigOut™ method digs up the rhizome roots and sifts every viable piece from the soil before returning the clean soil to the ground, saving the considerable costs associated with disposing of vast amounts of infected soil.
Cost of treatment
The cost of treatment will vary depending on the method you choose and the size of the area you need to treat.
Herbicide treatment is usually the cheapest option for treatment, but it can take several years to complete. The average price for a domestic herbicide treatment is around £2,000-£6,000 depending on the size of the infestation.
Full excavation and disposal are often more expensive and the average cost ranges from around £4000-£10,000. This again will depend on the size of the area affected.
How does Japanese Knotweed affect moving?
For those wishing to buy or sell a property, the discovery of knotweed doesn’t have to be a deal breaker.
The seller must declare on the TA6 Property Information form, which is completed as part of the conveyancing process, that the property has been affected by Japanese knotweed.
In these circumstances there is no need for the buyer to walk away from their dream home, but the seller will need to put a professional treatment plan in place and secure an insurance backed guarantee for a minimum of five years, preferably ten.
Once they have this guarantee, there should be no difficulties in obtaining mortgage finance and the property sale can proceed unhindered.
The 7-metre rule
In 2012, RICS introduced the 'seven metre rule' in their building surveys which shifted the focus for homeowners away from total eradication of the plant onto an 'appropriate level of control'.
Surveyors would sort the risk into four categories:
- Level 1 – Japanese Knotweed found on a neighbouring property more than 7 metres away from the boundary.
- Level 2 – Japanese Knotweed seen on neighbouring land within 7 metres of the boundary but not present on the property.
- Level 3 – Japanese Knotweed present on the property but is more than 7 metres from any structure. May be minor damage. Further investigation by a qualified/experienced person is required.
- Level 4 – Japanese Knotweed present within 7 metres of structures and/or is causing serious damage to these buildings and structures. Further investigation by a qualified/experienced person is required.
However, in 2022 RICS announced that they were changing the 7-metre rule as they determined that it was too vague. The new standard focuses more on impact rather than the distance of Japanese Knotweed.
This means they will not have to spend money on expensive treatments to remove the plant to increase the value of their home. This comes after new research suggests that Japanese Knotweed is not a major threat to the structure of buildings if it is controlled.
If Japanese Knotweed is found on a property it can be categorised in levels:
- Management Category A: Action – Japanese Knotweed is causing visible damage to the structure of the property. This will affect the value of the property due to repairs and management costs.
- Management Category B: Action – There is no visible damage to structures, but the plant may still prevent use or restrict access. This can still affect the value of the property as the Japanese Knotweed will need to be removed, but no repair costs will be needed.
- Management Category C: Action – Japanese Knotweed is present, but it has no affect or damage on the property. This should not affect the value of the property much.
Japanese knotweed indemnity insurance
There is also now a Japanese knotweed indemnity insurance policy available to anyone buying a property, enabling them to protect themselves from the risk of Japanese knotweed for a relatively small sum.
It covers the cost of treatment and repairs, as well as any legal defence expenses incurred should the knotweed spread to a neighbouring property. If the knotweed causes the property’s value to fall when the homeowner comes to sell, that loss is covered too, offering complete peace of mind to the homeowner.
Japanese Knotweed Heatmap
Below, find Environet's Japanese Knotweed Heatmap, showing where in the UK there are knotweed infestations. If your potential new home is in a knotweed area, it may be worth getting a Japanese knotweed survey.
Note: In 2019 Environet worked with a client to remove Japanese knotweed from their new property. The client started (and won) a legal case against the Chartered Surveyor who did not identify the plant during a Building Survey.
Japanese Knotweed FAQs
What is the 7-Metre rule for Japanese knotweed?
The 7-metre rule was introduced by RICS to identify and assess the damage and to a property because of Japanese Knotweed. It has since been replaced in 2022 with a method that focusses more on the impact of the plant rather than the distance.
What kills Japanese knotweed permanently?
Glyphosate-based herbicides can be used to treat Japanese Knotweed and can eradicate it if treated properly across two or three years of repeated treatment.
Is it worth buying a house with Japanese Knotweed?
The impact on the value of a property will depend on the extent of the Japanese Knotweed infestation and how close the plant is to structures.
The presence of the plant alone may not be a reason to avoid buying a property, but it is important to factor in costs of removing the plant and preventing it from causing future damage to the structure.
Is Japanese knotweed illegal in the UK?
Although it is not illegal to have Japanese Knotweed on your property, you may be prosecuted if you allow it to spread off your property and into the wild.
Updated May 2024